Skeletal muscle resistance is a major contributor to the hyperglycemia, hyperinsulinemia and dyslipidemia associated with type II diabetes and obesity. Recent work has implicated leptin, the adipocyte-derived hormone, in improving insulin sensitivity. Thus, leptin administration to leptin-deficient ob/ob mice corrects hyperglycemia and hyperinsulinemia, while elevating leptin in normal rats increases insulin sensitivity. Based on these observations the effects of leptin on the metabolic abnormalities of the high-fat fed rat, a model of diet-induced obesity that more closely resembles human obesity than monogenetic obesity models, were investigated. These studies, performed by the P.I. and discussed in this proposal, demonstrate that a gene therapy intervention that elevates plasma leptin levels reverses the skeletal muscle insulin resistance and other metabolic abnormalities associated with diet-induced obesity. However, the mechanisms underlying these effects are unknown. This proposal, therefore, focuses on identification of the mechanisms underlying leptin-induced reversal of skeletal muscle insulin resistance in diet-induced obesity. Three specific aims will test the hypotheses that skeletal muscle insulin resistance by leptin. These variables have been implicated in the pathogenesis of insulin resistance and the determination of muscle insulin sensitivity, and are altered by leptin. Identification of the mechanisms mediating leptin-induced reversal of muscle insulin resistance may serve as a platform for the rational design of pharmaceutical or genetic therapy of insulin resistance in human obesity and type II diabetes. Specific Aims: 1. To determine the role of altered lipid metabolism in mediating leptin-induced improvements in insulin sensitivity. 2. To determine the role of altered activity of the insulin signaling pathway in mediating leptin-induced improvements in insulin sensitivity. 3. To determine the role of altered metabolic gene expression in mediating leptin-induced improvements in insulin sensitivity.